The alkali metals form salt like hydrides by the direct synthesis at elevated temperature. The termal stability of these hydrides decreases in which of the following orders ?

Options

A. `NaH gt LiH gt KH gt RbH gt CsH`
B. `LiH gt NaH gt KH gt RbH gt CsH`
C. `CsH gt RbH gt KH gt NaH gt LiH`
D. `KH gt NaH gt LiH gt CsH gt RbH`

Answer

To determine the thermal stability of alkali metal hydrides, we need to analyze the relationship between the size of the cation and the covalent character of the hydrides formed. Here’s a step-by-step solution:

### Step 1: Identify the alkali metal hydrides
The alkali metals form the following hydrides:
- Lithium hydride (LiH)
- Sodium hydride (NaH)
- Potassium hydride (KH)
- Rubidium hydride (RbH)
- Cesium hydride (CsH)

### Step 2: Understand the trend in cation size
As we move down the group from lithium to cesium, the size of the cation increases. This means:
- Li+ < Na+ < K+ < Rb+ < Cs+

### Step 3: Apply Fajans' Rule
According to Fajans' rule, the larger the cation, the more covalent character the bond will have. This is because larger cations can polarize the electron cloud of the hydride ion (H-), leading to a more covalent character.

### Step 4: Determine the covalent character of the hydrides
Based on the size of the cations and Fajans' rule, the order of covalent character (and thus thermal stability) of the hydrides is:
- LiH (most covalent, highest thermal stability)
- NaH
- KH
- RbH
- CsH (least covalent, lowest thermal stability)

### Step 5: Write the order of thermal stability
Thus, the thermal stability of the alkali metal hydrides decreases in the following order:
**LiH > NaH > KH > RbH > CsH**

### Final Answer
The thermal stability of these hydrides decreases in the order: **LiH > NaH > KH > RbH > CsH**.

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